Contrast photometer

ABSTRACT

AN APPARATUS AND METHOD FOR INSTRUMENT DETERMINATION OF THE EFFECTIVENESS OF MILITARY OBSCURATION SYSTEMS BY THE DETERMINATION OF THE RATIO OF CLOUD REFLECTANCE TO COLD TRANSMITTANCE AS RELATED TO THE VISUAL CONTRAST DETERMINED BY A CONTRAST PHOTOMETER. THE METHOD AND APPARATUS CAN ALSO BE UTILIZED TO OBSERVE INDUSTRIAL SMOKE CLOUDS TO ASCERTAIN WHETHER OR NOT PARTICULATE MATTER OF THE CLOUD EXCEEDS THE MAXIMUM ALLOWABLE AMOUNT WITHIN POLLUTION CONTROL STANDARDS. AFOREMENTIONED INSTRUMENT DETERMINATION AND UTILITY IS ACCOMPLISHED BY AN APPARATUS AND METHOD WHICH EMPLOYS AN OPTICAL SYSTEM HAVNG AN OSCILLATING MIRROR MEANS THEREIN AND A PHOTOMULTIPLIER MEANS THEREIN? THE MIRROR MEANS BEING USED TO RECEIVE LIGHT WAVES FROM THE SOURCE BEING STUDIED BY THE PHOTOMETER AND TO TRANSMIT THE LIGHT WAVE TO A PHOTOMULTIPLIER MEANS TO CONVERT THE LIGHT WAVES INTO ELECTRICAL IMPULSES.

June 27, 1972 ROSENBLUM 3,672,781

CONTRAST PHOTOME'IER Filed March 3, 1970 5 Sheets-Sheet 1 INVENTOR Earl51 Rosenb/um 04 ,4 ATTOR Y June 27, 1972 E. s. ROSENBLUM CONTRASTPHOTOMETER Earl .5. Rosenb/um June 27, 1972 E. s. ROSENBLUM 3,

CONTRAST PHOTOMETER Filed March 3, 1970 5 Sheets-Sheet 3 Fig.

Ilk

"H II INVENTOR Earl S. Rosenblum June 27, 1972 E. s. ROSENBLUM 3,

CONTRAST PHOTOMETER Filed March 3, 1970 5 Sheets-Sheet 4 my? W a BYATTOR tzYJ June 27, 1972 Filed Marbh s, 1970 5 Sheets-Sheet 5PHUTOMUU/fi/[fi 5 SYA/(H/PO/VOUS MOTOR INVENTOR Earl 5. Rasenblum W- 5 oll 4' l United States Patent Ofli 3,672,781 Patented June 27, 19723,672,781 CONTRAST PHOTOMETER Earl S. Rosenblum, Lexington, Mass,assignor to the United States of America as represented by the Secretaryof the Army Filed Mar. 3, 1970, Ser. No. 16,115 Int. Cl. G01n 21/22,21/26 U.S. Cl. 356-201 9 Claims ABSTRACT OF THE DISCLOSURE An apparatusand method for instrument determination of the effectiveness of militaryobscuration systems by the determination of the ratio of cloudreflectance to cloud transmittance as related to the visual contrastdetermined by a contrast photometer. The method and apparatus can alsobe utilized to observe industrial smoke clouds to ascertain whether ornot particulate matter of the cloud exceeds the maximum allowable amountwithin pollution control standards. Aforementioned instrumentdetermination and utility is accomplished by an apparatus and methodwhich employs an optical system having an oscillating mirror meanstherein and a photomultiplier means therein; the mirror means being usedto receive light waves from the source being studied by the photometerand to transmit the light waves to a photomultiplier means to convertthe light waves into electrical impulses.

DEDICATORY CLAUSE The invention described herein may be manufactured,used, and licensed by or for the Government for governmental purposeswithout the payment to me of any royalty thereon.

SPECIFICATION My invention, which was conceived and reduced to practiceunder U.S. Government contract DA18-03S- AMC706(A) between theDepartment of the Army and GCA Corporation, relates to an apparatus andmethod for determining the ratio of cloud reflectance to cloudtransmittance by observing a military obscuration system, such as achemical or an industry generated smoke cloud, with a contrastphotometer by a means of a remote measurement technique involving theuse of a pair of reflectance contrast targets located on the side of thecloud opposite to the photometer; one of the target pair beingsubstantially black and the other being substantially white.

The prior art method employed to evaluate the obscurance provided by amilitary obscurance system was visual observation by a human. Suchobservation, of course, incorporated the human error and relied on humanjudgment in ascertaining the degree of obscurance. Regarding industrialsmoke clouds, particulate pollutants were measured in the prior art bynephelometers and transmissivity photometers or by collecting theparticulate matter and measuring the physical characteristics thereof.

The principal object of my invention is to provide a reliable andeffective method and apparatus for measuring the obscurance provided bya military obscurance system or the amount of particulate pollutant inan industrial smoke cloud.

Other objects of my invention will be obvious or will appear from thespecification hereinafter set forth.

FIG. 1 is a view showing the utility of my contrast photometer.

FIG. 2 is a pictorial view of my contrast photometer.

FIG. 3 is a sectional view of my contrast photometer.

FIG. 4 is a view of the mechanically mounted embodiment of theoscillating mirror of my contrast photometer.

FIG. 5 is a view of the electro-magnetically operated oscillating mirrorof my contrast photometer.

FIG. 6 is a block diagram of the electronic circuitry of my own contrastphotometer.

FIG. 7 is an electrical circuit diagram of the electronics of mycontrast photometer.

FIG. 8 is an electrical diagram of the mirror driveroscillator-reference(chopper) section of my contrast photometer.

FIG. 9 is a block diagram of the heater arrangement of my contrastphotometer.

FIG. 10 is an electrical circuitry diagram of the typical operationalamplifier wiring arrangement for my contrast photometer.

FIG. 11 is a view of the plug connection for the heaters of my contrastphotometer.

FIG. 12 is a view of the plug connection for the power supply of mycontrast photometer.

FIG. 13 is a schematic diagram showing the mirror drive in a typicalview of the image plane for my contrast photometer.

My invention as shown in FIGS. 1 to 13 will now be described in detailas follows.

The conventional method for evaluating effectiveness of obscurationagents involves finding a number called the Total Obscuring Power (TOP)for the agent. The TOP is measured in a laboratory experiment whichgives results which do not always correspond to the obscuringeffectiveness of munitions deployed in the field.

There are a number of reasons for the lack of agreement between TOP andobscuring effectiveness; most important of which is the fact that, whileTOP varies with the ability of an agent cloud to attenuate light signalspassing through it, it is not a good measure of the clouds ability toscatter sunlight and other light coming from all directions into theeye.

It has been known for several decades that visual detection of an objectdepends on the brightness difference between the object and itsbackground and also on the average brightness seen by the eye. If theeye sees a low average brightness, a target only slightly brighter ordarker than its background might be detected. On the other hand, if theeye sees a high average brightness, a target only slightly brighter ordarker than its background could not be detected.

Thus, at one extreme, an agent which does not attenuate light signals atall, does not decrease target-background brightness differences,andscatters considerable light into the line of sight; thereby increasesthe average brightness seen. Such an agent would have a TOP of zero, butit might obscure appreciably under certain conditions. At the otherextreme, an agent which attenuates signals without scattering might havea large TOP, but it would obscure very inefficiently.

Since agents both attenuate and scatter light, a method and/or apparatusto determine obscuration effectiveness should account for bothproperties; preferably in a form related simply and directly to theagents ability to prevent detection by observers.

My apparatus, method, and system measures a property, called theobscurance of an agent or industrial smoke cloud, which does have theaforementioned characteristic of light attenuation and scattering.Obscurance measures directly the ability of a cloud to restrictvisibility. A munition which generates a smoke cloud having a givenobscurance will decrease the detectability of any target moreeffectively than will another munition generating a smoke cloud having asmallr obscurance, when the two munitions and generated smoke clouds aretested under the same conditions.

It is possible that Munition A may obscure more effectively thanMunition B under one set of conditions, while under other conditions,such as with the sun making a different angle with the line of sight,Munition B may obscure better. This difference is possible because cloudparticles of different materials or different size distributions havedifferent scattering properties as the directions of illumination andview vary. Measurements of obscuranee detect the aforementioneddifference, while TOP does not.

As indicated above, mans ability to visually detect an object depends onthe objects brightness, the angle it subtends at the eye of theobserver, the brightness of the background surrounding it, and the levelof illumination to which the eye is adapted. Furthermore, the brightnessdifference between the object and background of a firstobject-background pair may be the same brightness between the object andbackground of a second objectbackground pair and yet the two objectswill not be equally visible if the average brightness of the firstobject-background pair is darker than the average brightness of thesecond object-background pair; the darker the objectbaokground pair willbe more visible.

The important factor is the relative contrast between the object andbackground. If the brightness of a target is B, and the brightness ofanother target or the background to the first target, is B, theirrelative contrast is defined as universal contrast C and is determinedby the equation:

wherein For target-background pairs having a given target size, at agiven eye-adaptation level, all pairs having the same value of C or mwill be equally visible. Near the lower limit of detectability, anobject slightly darker than its background will be equal in visibilityto another object slightly brighter than its background, if the absolutevalue of the contrast is the same in both cases, even though it isnegative in one case and positive in the other. It is to be noted that,for a given contrast constant, the difference in brightness between anobject and background is increased if the background becomes brighter.

*In order to obscure effectively, an obscuration agent must decrease thetarget-background brightness difference and increase overall brightness.

Brightness reduction by a smoke cloud is accomplished by attenuation ofthe light passing through it. All sources of white light have theirapparent brightness decreased in the same proportion. If the agentproduces a truly black smoke, the smoke cloud attenuates without addingscattered light. In such a smoke cloud, the difference between thetarget and background brightness is decreased, but the backgroundbrightness itself is decreased in the same proportion, and there may beno change or an increase in contrast. Unless the whole visible fieldbecomes dark enough to cause a significant change in eye-adaptationlevel, the smoke will give no reduction in detectability.

On the other hand, a white smoke, which attenuates light passing throughit, also scatters light into the eye from all sources of illumination,particularly the sun or moon. Reduction in brightness difference isaccompanied by an increase in apparent brightness of both the target andbackground which results in a significant decrease of contrast andvisibility.

As stated above, the minimum detectable contrast is a function of theobject angular size and eye-adaptation level. Such detectable contrastis called the limen and will be indicated by the symbols G or m Undermost condi- 4 tions of viewing, the limen is a small number. Thisimplies that E and B are nearly the same in aforementioned Equations 1and 2 and also that EZ B in aforementioned Equation 3, from which itfollows that Equation 4 gives a convenient means of relating testresults .given in terms of the modulation contrast m, to the results oftests with human observers, which are nearly always given in terms ofthe universal contrast C.

The most significant aspect of my invention is that, for a given set ofsource-target-agent-observer locations, at any instant of time, there isa single evaluation, parameter, the obscuranee, for each munition whichpermits an absolute evaluation of the contrast-reducing ability. Usingthe obscuranee parameter, munitions can then be ranked according to suchparameter, and the effectiveness of each determined under various fieldconditions. Similarly, industrial smoke can be ranked according to thedegree of air pollution being generated, or the degree of atmosphericturbulence measured in micro-meteorological studies by using a smokecloud as an indicator means.

Contrast-reducing ability can be expected to change with differentrelative positions of my apparatus used to determine it, and there willbe a set of obscurances for each munition. Obscurance will also changeas a result of changing meteorological conditions and varying particlesize distributions. Accordingly, the relative ranking of munitions andindustrial smoke clouds may be different for different cloud particulategeometries and different weather conditions.

Obscurance, as stated above, is determined from measurements ofcontrast, or equivalently, from measurements of visual brightness.Instruments for measuring brightness are termed photometers, and a largevariety of photometers, of two general classes, have been described inthe literature; the two classes being visual and photoelectricphotometers. Measurements by visual photometers consist of visualcomparison of an image of the desired field with an image of a fieldhaving a known brightness, whereas photoelectric photometers giveelectric outputs having a magnitude which can be related to thebrightness of the field viewed by the photometer.

Brightness measurements with a visual photometer is an inherently slowprocess, because the determination of contrast requires measurement ontwo fields and gives meaningful results only if the two measurements aremade nearly simultaneously. Since the brightness of both targets viewedin field tests of obscuration agents can be expected to vary rapidly andcontinuously throughout the tests, visual photometers would not beexpected to give useful measurements.

While photoelectric photometers have a wide range of response times,such photometers can be selected to respond rapidly to varying signals.In addition, photoelectric photometers have the additional usefulproperty of giving an output which can be recorded continuously in theform of a permanent record from which brightness, contrast, andobscuranee can be calculated at a later time.

For the aforementioned reasons, the design of my apparatus to determinesmoke cloud obscurance was based on a photoelectric photometer. Aphotomultiplier was selected, because it has a fast response and anamplified output which is linear with the intensity of the incidentlight, requires only a simple power supply and can be purchasedinexpensively with a spectral response that is matched readily to thatof the human eye.

In solving the problem of making contrast measure ments between a targetpair having rapidly-varying brightness, two systems can be utilized asfollows: (1) Two targets of different inherent reflectivities may bepresented alternately to a photometer having a fixed line of sight or(2) Two fixed adjacent targets may be viewed alternately by; photometerhaving an alternating or rotating line of Slg t.

Above system (1) has the advantage that the photometer design isrelatively simple, and a fixed line of sight minimizes spuriousmodulations; such modulation being caused by turbulent agent-cloudbrightness variations. On the other hand, system 1) requires anindependent source of target movement and a synchronizing signal whichoriginates at the target for optimum detection sensitivity. Sincetarget-detector distances may be quite large in the field use, thisinvolves additional power and signal cables that may be quiteinconvenient. Also, the photometer is designed with only a moderatelysmall fieldof view so that it can have adequate sensitivity in a compactunit without sophisticated or expensive components. At field use rangesof hundreds of feet, this system requires rather large targets, andlarge rotating or oscillating targets in the field necessitates massive,cumbersome, and powerconsuming construction.

Above system (2), my preferred embodiment, permits the use of simple,stationary targets, and all power and synchronizing signals are locatedat one station. The optical axis of the photometer of system (2) can beoscillated or rotated in a number of manners, all of which can beaccomplished with negligible power. The oscillation of the line of sightcan result in a residual small signal modulation caused by opticalasymmetry inside the instrument and in the transmission of differenttransmissive and reflective properties of the smoke cloud along the twolines of sight. Residual small signal modulation caused by opticalasymmetry was minimized by careful design.

None of the commercially available photometers has a satisfactorycombination of speed, sensitivity, spectral response, field of view,oscillating optic axis, compactness, and low cost. Therefore, myapparatus was conceived and reduced to practice to overcome the abovestated problems and to satisfy the long felt need for an apparatus tomeasure military smoke cloud obscurance and industrial smoke cloud airpollution.

In using my apparatus, as shown in FIG. 1, the photom-. eter, as shownin FIGS. 1, 2, and 3 at 1, is sited and focused on contrasting targets 3and 4 by means of eyepiece 6, shown in FIG. 3; target 3 beingsubstantially white and target 4 substantially black. Targets 3 and 4are located on the side of smoke cloud 2, generated by a conventionalsmoke munition 5, opposite to photometer 1. Any conventionalmulti-recorder, such as a six channel multiplexed routine automatic dataconversion portable digital recorder, is connected to a terminal 7,electrical power is supplied to the synchronous motor for operating theoscillating mirror by connecting plug 8 to any conventional AC outletand placing the mirror oscillator in operation. Photomultiplier 18 issupplied by a conventional high voltage supply through. terminals 50 byturning switch 10 to the on position, and the sensitivity of thephotomultiplier is controlled by rheostat 9.

Light passes through elbow telescope 11 which incorporates the opticsshown in insert A in FIGS. 3, 4, and 5, a conventional light filter 12,such as a Wratten filter, an objective lens 13, filter wheel 14 having aplurality of filters mounted thereon for operation at different lightlevels, switch means 29 to select an appropriate filter, a housing toaccommodate optical system components, eyepiece 6, field stop 17, andphotomultiplier 18.

A number of alternative mechanisms for alternating the field of view tobe seen by the photomultiplier can be utilized as follows:

( l) Rotating optical wedge (2) Tuning-fork chopper (3) Rotatingeccentric mirror (4) Oscillating mirror Mechanisms (1) and (2) permit astraight-through optical system, while mechanisms (3) and (4) require abend in the optical axis which will result in a different sensitivity tohorizontally and vertically-polarized incident light.

However, it is somewhat more complicated mechanically to construct aninstrument with mechanisms (1) and (2). Mechanisms (1) and 3) cause thefield of view to rotate which requires a target that is essentiallycircular, and mechanisms (2) and (4) give an oscillating field of viewwhich requires a target that is extended horizontally but notvertically, a somewhat simpler geometry for field use. Mechanism (4) ismy preferred embodiment, and the heart of this configuration is theoscillating mirror. Mechanism (4) consists of a front surface mirror 19rigidly mounted on a stainless steel torsion bar 20 which is restrainedat its extreme ends by mounts 27. Mounted on the mirror is a small nylonrod, not shown in the drawing, which bears against a rotating eccentricplate 21 driven by a synchronous motor 22. To minimize dynamic andfriction problems, a 300 r.p.m. motor was selected which gives a signalwhich oscillates at 5 Hz. The position of the eccentric plate 21 on thedriving shaft 23 can be adjusted to set the mirror 19 to an angle of 45degrees with the axis of the telescope 11 which turns the light pathapproximately degrees; the preloading of the torsion bar is alsogoverned by this adjustment. Telescope objective lens 13 and mirror 19focus an image on the image plane 24, in which is machined an aperture25 which defines the field of view through the relation:

Aperture diameter Field of view in TELdlZLnS- W Light passing throughaperture 25 diverges and falls on the photocathode 26. The cone of lightoscillates slightly as the mirror oscillates through an angle determinedby the eccentricity of the eccentric plate so that the images of the twotargets fall on different portions of the photocathode, which may havedifferent sensitivities. To minimize the effects of sensitivityvariation, photomultiplier 18 is mounted at such a distance from theaperture that there is considerable overlap between the cones of lightto cover a large enough area and! still fall entirely within the boundsof the photocathode. The preferred embodiment of mechanism (4) is shownin FIGS. 4 and 5; FIG. 4 consisting of two views, the upper view being atop view and the lower view the front view. Alternative to using asynchronous motor drive for mirror 19, a conven tional electro-magneticdrive can be used as shown in FIGS. 5 and 8 wherein structures 28 areelectro-magnetic coils.

An index wheel, not shown in the drawing, can be mounted on the motorshaft to allow the operator to position the mirror at a known point sothat the instrument can be accurately pointed at a target or standardrefiectance surface during calibration or for brightness measurementsbefore a field test.

The photomultiplier has a conventional S4 spectral response and aconventional light filter, such as a Wratten No. 106 filter, not shownin the drawing, mounted behind field stop 17 that gives an overallspectral response closely matching that of the photop-ic eye.

Eyepiece 6, not shown in detail in the drawing, consists of a simplelens and a mirror to form a virtual image of the image plane for thehuman eye; the image plane being painted white; the field stop appearingas a black dot, accurately defining the area viewed by thephotomultiplier. With a light filter behind the image plane, a somewhatbrighter image is seen in the eyepiece.

While the design parameters used in the optical system of my apparatusare as follows:

(1) Objective lens, 2 /2 in. dia., 7 in FL.

(2) Field stop aperture, 0.025 in. dia.

(3) Angular field of view:

Calculated=0.02S/7 =.0036 rad=l2.5 min. Measured=.0045 rad=l5 min.

(4) Angular shift of image=26 min.

(5) Rate of oscillation of field of view=5 Hz.

these parameters are adjustable within the skill of the art to suit anygiven application for my device and method.

The oscillating mirror in the photometer causes first the white portionof target 3 to be imaged on the photocathode, then the black portion oftarget 4 which results in two signals being produced; a peak signal anda modulation signal. The peak signal, designated DC, is proportional tothe maximum brightness detected by the photometer; whereas themodulation signal, designated AC is proportional to the logarithm of thedifference between the maximum and minimum brightness detected when thephotometer scans across the target. If B is the maximum brightness and Bis the minimum brightness, then DC=KB and out= ln[ max min) 1 where cand d are constants applicable to the particular logarithmic amplifierused as set forth in subsequent equations and k is the calibrationconstant of the photometer.

The modulation signal, AC is linearly proportional to the brightnessdifference m= u-lax min) P( 0+ 1 out) =e( 0+ 1 out) wherein a =d/c and a=1/c.

a and a are the calibration constants of the logarithmic amplifier ofthe contrast photometer and are determined from a least squares fit tothe calibration curve. The contrast is defined as max. i-nin.) max.+min.

which can then be written in terms of output voltages, DC, and AC as 2DCAC With m the inherent or background contrast, and m the contrast withthe obscuration agent present, the modulation contrast transmittance 1is defined by m m max. min. x maX.' min. o

A0... 1 2120-110... x in wherein the subscripts o and x refer tomeasurements of the target performed without an obscuring agent and withan obscuring agent, respectively.

Finally the obscurance R is given by wherein R is the average inherentreflectivity of the test target. The ratio of the obscurance to theaverage refiectivity, R/R is then the quantity calculated to determinethe effectiveness of the obscuration agents or pollution caused byparticulate pollutants.

Signal processing by the electrical circuitry shown in FIGS. 7 and 8 ismost clearly understood by considering the schematic diagram presentedin FIG. 6. Light waves 30 passing through lens 13 and striking mirror 19are transmitted through aperture 25 of field stop 17 to photomultiplier18. Photomultiplier 18 sensitivity is matched to the intensity range ofthe input signals of the light wave by a conventional matching circuit31 through varying a high voltage supply 32 by means of input voltageadjustment 33. The output signal from photomultiplier 18 is transmittedto pre-amplifier 34 and the peak to peak value of the wave form and thebase to peak value of the wave form of the pre-amplifier output signaldetected as the peak signal and modulation signal respectively. The peaksignal is transmitted to peak signal detector means 35 and subsequentlyto output amplifier 36 for amplifiication to a suitable level to berecorded by the previously discussed digital recorder connected toterminals 37. The modulation signal is filtered by filter means 38 andtransmitted to log amplifier 39 to be compressed logarithmically toproduce a linear output over a signal range of 1000:1. Amplifier 39signal is filtered by active filter means 40 and transmitted to phasesensitive detector means 41 and to output amplifier 42 for amplificationto a suitable level to be recorded by the previously discussed digitalrecorder connected to terminals 37. The modulation signal is detected bymeans 41 synchronously with a reference signal from a scanner supplyhaving a phase shift means 43 and a chopper driver means 44 connectedtherein. Mirror means 19 is operated by oscillator driver means 45.Although both the peak and modulation signals can be recorded in analogform on a simple dual-track recorder, the signals are also suitable fordigital recording.

The time constants and rates of data acquisition depend on therequirements of the evaluation procedure. The basic data acquisitionrate must be higher than the maximum significant frequency contributionto the variance of the contrast or obscurance, if the total variance isto be measured reliably.

The logarithmic amplifier in the circuit is quite temperature sensitive.Since small gain changes due to ambient temperature variation produce,in a circuit with logarithmic response, output changes corresponding tolarge apparent changes in the input, and since the entire optical andelectronic package is mounted in a single enclosure, a thermostaticallycontrolled heater circuit, as shown schematically in FIG. 9 andincluding thermostat means 46, relay means 47, and heater means 48 and49, is provided inside the entire enclosure to permit operation of theelectronic system at approximately 22 C. It is well within the skill ofthe art to adjust the heater circuitry to suit any necessary operatingtemperature requirement for any given application.

The circuitry described above and shown schematically in FIGS. 6 and 9is shown in detail in FIGS. 7, 8, and 10 to 12. It is considered thatone of ordinary skill in the art will fully understand the operation asshown in FIGS. 7, 8, and 10 to 12. in the light of the above schematicdescription and no further explanation is necessary. While eachcomponent in the electronic circuitry of my apparatus is conventional,the circuitry design is novel, unobvious, and solves the aforementionedprior art problems. The optical and electro components of my inventioncan be changed and modified within the skill of the art to adapt theinvention to any visible or invisible electro-magnetic wave application.

It is obvious that other modifications can be made of my invention, andI desire to be limited only by the scope of the appended claims. 1

I claim:

1. An apparatus for measuring the degree of obscuration created by asmoke cloud selected from the group of smoke clouds consisting ofchemical smoke clouds and industrial smoke clouds comprising an opticalsystem fixedly mounted within a telescope means, said optical systemcomprising a light filter means located adjacent to the large diameterend of the telescope, an objective lens means located adjacent to thefilter means and an eyepiece means, an oscillating mirror means locatedadjacent to the eyepiece means and behind said lens means, aphotomultiplier means and an aperture located in a field stop means,said mirror means being positioned to receive light waves from a distantlight source and to transmit the light waves to said photomultipliermeans through said aperture means, said eyepiece means located means,said eyepiece means serving to focus the optical system on said lightsource, a filter wheel means having a plurality of filters mountedthereon, means for selecting from said plurality of filters apredetermined filter for a predeter mined light source, said filterwheel means being located between the oscillating mirror means and theaperture means located in the field stop means, said field stop meansbeing located between the filter wheel means and said photomultipliermeans; said photomultiplier means converting light waves received fromthe optical system into electrical impulses; means to measure theelectrical impulses from the photomultiplier means as a peak signal anda modulation signal; a first amplifier means to amplify the peak signalfor recordation; a filter means to filter the modulation signal; a logamplifier means to logarithmically compress the modulation signal; anactive filter means to filter the logarithmically compressed modulationsignal; a second amplifier means to amplify the modulation signal forrecordation and a recorder means to record the peak signal and themodulation signal.

2. The apparatus of claim 1 wherein the oscillating mirror means has amounting system comprising a stainless steel torsion bar rigidly mountedon the mirror, said torsion bar being restrained at the outer ends bymount means fixedly attached to a frame means of the telescope means;and a nylon rod means fixedly mounted on the mirror means, said nylonrod means bearing against a rotating eccentric plate to oscillate themirror means.

3-. The apparatus of claim 2 wherein the eccentric plate is driven by asynchronous motor.

4. The apparatus of claim 2 wherein the eccentric plate is driven by anelectro-magnetic circuit means.

5. The apparatus of claim 1 wherein the photomultiplier means has asensitivity matched to the intensity range of the input signals of alight Wave, said matching being produced by a circuit comprising amatching circuit means; a high voltage means; and a voltage adjustmentmeans.

6. The apparatus of claim 1 wherein the means to measure the electricalimpulses from the photomultiplier means is a resistance-capacitance peakdetector circuit and a phase-sensitive chopper detector circuit tomeasure the peak signal and the modulation signal respectively.

7. The apparatus of claim 1 wherein the recorder means is a six channelmultiplexed automatic data conversion digital recorder.

8. A system for measuring the degree of obscuration created by a smokecloud selected from the group of smoke clouds consisting of chemicalsmoke clouds and industrial smoke clouds comprising apparatus having anoptical system fixedly mounted within a telescope means, said opticalsystem comprising a light filter means located adjacent to the largediameter end of the telescope, an objective lens means located adjacentto the filter means and an eyepiece means, an oscillating mirror meanslocated adjacent to the eyepiece means and behind said lens means, aphotomultiplier means and an aperture located in a field stop means,said mirror means being positioned to receive light waves from a distantlight source and to transmit the light waves to said photomultipliermeans through said aperture means, said eyepiece means located at theend of the telescope means opposite to the lens means, said eyepiecemeans serving to focus the optical system on said light source, a filterwheel means having a plurality of filters mounted thereon, means forselecting from said plurality of filters a predetermined filter for apredetermined light source, said filter wheel means being locatedbetween the oscillating mirror means and the aperture means located inthe field stop means, said field stop means being located between thefilter wheel means and said photomultiplier means; said photomultipliermeans converting light waves received from the optical system intoelectrical impulses; means to measure the electrical impulses from thephotomultiplier means as a peak signal and a modulation signal; a firstamplifier means to amplify the peak signal for recordation; a filtermeans to filter the modulation signal; a log amplifier means tologarithmically compress the modulation signal; an active filter meansto filter the logarithmically compressed modulation signal; a secondamplifier means to amplify the modulation signal for recordation and arecorder means to record the peak signal and the modulation signal and apair of contrasting target members for reflecting light waves whichresult in the peak signal and the modulation signal, one member of thetarget pair being substantially White and the other member of the targetpair being substantially black.

9. A method for measuring the degree of obscuration created by a smokecloud selected from the group of smoke clouds consisting of chemicalsmoke clouds and industrial smoke clouds comprising the steps offocusing a telescope means on a pair of contrasting target means,locating said target means on a side of the smoke cloud opposite to thetelescope means; oscillating a mirror means to selectively reflect lightwaves emanating from each member of the pair of target means through thesmoke cloud to the optical system to detect the transmissive andreflective properties of the smoke cloud; transmitting the light wavesfrom the optical system to a photomultiplier means to convert the lightwaves into electrical impulses; matching the photomultiplier meanssensitivity to the intensity range of the light Wave signals; splittingthe electrical impulses into two components, one component being a peaksignal and the other component being a modulation signal; detecting thepeak signal to determine the peak signal characteristics of the smokecloud; logarithmically compressing the modulation signal; detecting themodulation signal to determine the modulation signal characteristics ofthe smoke cloud; recording the peak signal and the modulation signal;and determining the obscurance by means of the equation O (1-rm) whereinR is the obscurance, R, is the average inherent reflectivity of thetarget, and

wherein AC, is the modulation signal, DC is the peak signal, andsubscripts 0 and x are measurements of the target without a smoke cloudintervening and with a smoke cloud intervening respectively.

References Cited UNITED STATES PATENTS 2,198,971 4/1940 Neufeld 356-208X RONALD L. WIBERT, Primary Examiner F. L. EVANS, Assistant ExaminerU.S. Cl. X.R.

